It is highly desirable for tires to have good wet skid resistance, low rolling resistance, and good wear characteristics. It has traditionally been very difficult to improve a tire's rolling resistance without sacrificing its wet skid resistance and traction characteristics. These properties depend to a great extent on the dynamic viscoelastic properties of the rubbers utilized in making the tire.
In order to reduce the rolling resistance of a tire, rubbers having a high rebound have traditionally been utilized in making the tire's tread. On the other hand, in order to increase the wet skid resistance of a tire, rubbers which undergo a large energy loss have generally been utilized in the tire's tread. In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For example, various mixtures of styrene-butadiene rubber and polybutadiene rubber are commonly used as a rubbery material for automobile tire treads. However, such blends are not totally satisfactory for all purposes.
U.S. Pat. No. 4,843,120 discloses that tires having improved performance characteristics can be prepared by utilizing rubbery polymers having multiple glass transition temperatures as the tread rubber. These rubbery polymers having multiple glass transition temperatures exhibit a first glass transition temperature which is within the range of about -110.degree. C. to -20.degree. C. and exhibit a second glass transition temperature which is within the range of about -50.degree. C. to 0.degree. C. According to U.S. Pat. No. 4,843,120 these polymers are made by polymerizing at lease one conjugated diolefin in a first reaction zone at a temperature and under conditions sufficient to produce a first polymeric segment having a glass transition temperature which is between -110.degree. C. and -20.degree. C. and subsequently continuing said polymerization in a second reaction zone at a temperature and under conditions sufficient to produce a second polymeric segment having a glass transition temperature which is between -20.degree. C. and 20.degree. C. Such polymerizations are normally initiated with an organolithium initiator and are generally carried out in an inert organic solvent.
U.S. Pat. No. 5,137,998 discloses that terpolymers of styrene, isoprene, and butadiene which exhibit multiple viscoelastic responses result from terpolymerizations of styrene, isoprene, and 1,3-butadiene in the presence of an alkali metal alkoxide and an organolithium initiator. By utilizing this technique, such terpolymers which exhibit multiple glass transition temperatures can be prepared in a single reaction zone. The SIBR (styrene-isoprenebutadiene rubber) made by the technique of U.S. Pat. No. 5,137,998 offers an outstanding combination of properties for utilization in making tire tread rubber compounds. For example, utilizing such SIBR in tire tread compounds results in improved wet skid resistance without sacrificing rolling resistance or tread wear characteristics.
It is known in the art that 3,4-polyisoprene can be used in tire tread compounds to improve tire performance characteristics, such as traction. Polar modifiers are commonly used in the preparation of synthetic polydiene rubbers which are prepared utilizing lithium catalyst systems in order to increase their vinyl content. Ethers and tertiary amines which act as Lewis bases are commonly used as modifiers. For instance, U.S. Pat. No. 4,022,959 indicates that diethyl ether, di-n-propyl ether, diisopropyl ether, di-n-butyl ether, tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, trimethylamine, triethylamine, N,N,N',N'-tetramethylethylenediamine, N-methyl morpholine, N-ethyl morpholine, and N-phenyl morpholine can be used as modifiers.
U.S. Pat. No. 4,696,986 describes the use of 1,2,3-trialkoxybenzenes and 1,2,4-trialkoxybenzenes as modifiers. The vinyl group content of polydienes prepared utilizing Lewis bases as modifiers depends upon the type and amount of Lewis base employed as well as the polymerization temperature utilized. For example, if a higher polymerization temperature is employed, a polymer with a lower vinyl group content is obtained (see A. W. Langer; A. Chem. Soc. Div. Polymer Chem. Reprints; Vol. 7 (1), 132 [1966]). For this reason it is difficult to synthesize polymers having high vinyl contents at high polymerization temperatures utilizing typical Lewis base modifiers.
Higher temperatures generally promote a faster rate of polymerization. Accordingly, it is desirable to utilize moderately high temperatures in commercial polymerizations in order to maximize throughputs. However, it has traditionally been difficult to prepare polymers having high vinyl contents at temperatures which are high enough to attain maximum polymerization rates while utilizing conventional Lewis bases as modifiers.
U.S. Pat. No. 5,231,153 reports that compounds having the following structural formulae can be used as modifiers in the synthesis of polydienes: ##STR2## wherein n represents an integer within the range of 3 to 6, and wherein R, R.sup.1, and R.sup.2 can be the same or different and represent alkyl groups containing from 1 to 10 carbon atoms, aryl groups containing from 6 to carbon atoms, or hydrogen atoms.
U.S. Pat. No. 5,231,153 reports that these modifiers remain stable at conventional polymerization temperatures and lead to the formation of polymers having high vinyl contents at such temperatures. Accordingly, they can be used to promote the formation of high vinyl polymers at temperatures which are high enough to promote very fast polymerization rates.
Japenese Patent 5255540 to Noriyuki which is assigned to Toyo Tire & Rubber discloses a rubber composition for pneumatic tire treads which is reported to provide improved wear and skid resistance. This tire tread compound is comprised of a styrene-isoprene copolymer, a styrene-butadiene copolymer, and carbon black. These compositions contain 50 to 150 parts by weight of carbon black per 100 parts by weight of the rubbers in the compound.